The expanding cruise ship industry estimates more than 27 million people worldwide will this year holiday on the giant vessels, some of which are longer than an aircraft carrier, often dwarfing the landscapes of the ports they dock in. Environmentalists warn that they also leave behind them a noxious blend of particulates and gases that represent a serious health risk to the populations of the locations they visit, the tip of the iceberg of the problem of pollution caused by maritime traffic. Dorothée Moisan reports on the dirty side of sea travel.